While I'm here, I have another question that I don't know if has ever been adressed. I know the show has been on for 50 years. However, has it been established how long each incarnation of the Doctor has survived? If it's been real time, then the first Doctor would have lasted nearly 900 years, but the other 11 would have each lasted a fraction of 50 years each, which really isn't all that long. Of course, with all that pesky time travel, is there any way of knowing how long each particular Doctor lasted before regenerating?
NuWho has been pretty consistent with his aging. Classic Who was all over the map, with the Doctor Quoting himself as 900 something and then a Regeneration or two later claiming himself to be 700 something.
So, no, it's not really nailed down, especially if taken from the entire 50 year history of the show.
The First Doctor's body wore out from age, so, it's a good bet he lived a pretty long life before we saw him onscreen. The Second Doctor was force Regenerated, and the rest have all Regenerated from a Mortal Peril. Anytime you see The Doctor take off in his TARDIS alone, even if he returns a second later, it's possible he could've been out there having adventures for 100s of years in that 1 second of show time, but, if we always see a Doctor get into the TARDIS and travel with a Human Companion, we can assume how much time has passed